Electron-impact-induced fragmentation of long-lived C<inf>14</inf>H<inf>14</inf>^•+ ions from dibenzyl sulphone and 1,2-diphenylethane

Gaseous dibenzyl sulphone radical cations (1•+) expel SO2, forming C14H14•+ ions, whose fragmentation is identical with that of the radical cations of 1,2-diphenylethane (2•+). Metastable ions [1 - SO2]•+ and 2•+ eliminate predominantly benzene to give C8H8•+, in competition with the well-known benzyl cleavage. The [M - SO2]•+ ions from deuterium-labelled isotopomers of 1 react in exactly the same way as 2•+ ions bearing the label in corresponding positions (e.g. C6H5CD2SO2CD2C 6H5•+ and C6H5CD2CD2C6H 5•+). Benzene loss is initiated by migration of a benzylic H atom, but does not occur by simple 1,2-elimination; rather, the C6H6 molecule incorporates an H atom from the other ring. Both fragmentation reactions are preceded by slow interannular hydrogen exchange, which involves preferentially the ortho positions of the rings; however, this exchange is coupled with a relatively fast intra-annular hydrogen ring-walk by which the hydrogens at the meta and para positions of the rings participate in the interannular exchange. The data suggest that the extrusion of SO2 from ionized dibenzyl sulphone generates a complex of a benzyl cation and a benzyl radical, which recombine by bond formation between the benzylic α carbon atoms, but possibly also by electrophilic attack of C6H5CH2•+ at the various sites of C6H5CH2•. © 1997 Elsevier Science B.V.